(1) Field of the Invention:
This invention relates to an apparatus for measuring, by a probe, electrical characteristics of a semiconductor device formed on a wafer.
(2) Description of the Prior Art:
Apparatus often called "wafer probing machines" or "probers" have been employed to electrically measure characteristics of semiconductor devices formed on wafers or those of wafers per se. In each of such apparatus, a probing needle is usually brought into contact with a target portion of a semiconductor device to collect information or data on one or more electric characteristics of the semiconductor device at the portion. A microscope is provided in combination with the probing needle. By automatically or visually shifting the microscope and probing needle relative to the wafer, the probing needle is brought into contact precisely to the target position for measurement thereof.
FIG. 5 is a perspective view showing a conventional wafer probing machine. A wafer 1, which is to be measured, is held under vacuum suction on a sample stage 2 which is movable along three axes, X, Y and Z. An electrically-conductive needle 4 made of a material such as tungsten or the like and provided on the lower surface of an arm 3 arranged above the stage 2 is then brought into contact with a desired portion of the wafer 1. A microscope 5 is incorporated in the arm 3 in such as way that the tip of the needle 4 is contained in its field of view, preferably, almost at the center. While watching the positions of the tip of the needle and the device on the wafer, the stage 2 is moved in the X-Y directions until a desired portion of the device is aligned with the tip of the needle. Thereafter, the stage 2 is raised along the Z-axis so as to bring the desired portion of the device into contact with the tip of the needle 4. Information or data collected as a result of the above contact are then input to a processing unit 8 through wires 6,7.
Such wafer probing machines are well-known and several kinds of apparatus have been put on the market, including for example an apparatus with a plurality of needles arranged in a pattern corresponding to the layout of plural terminals in a semiconductor device, an apparatus with a relay matrix interposed between a probing needle and a plurality of processing units designed respectively for plural kinds of characteristics to be measured, etc.
These conventional apparatus generally take horizontal forms, that is, the wafer 1 is set on the horizontal stage 2 as depicted in FIG. 5. They are hence accompanied by the following drawbacks.
First of all, a structure such as the microscope 5 is arranged above the wafer 1. There is thus a danger that upon focusing, dust such as fine metal particles could drop onto the surface of the wafer 1 and might stick there. Such dust interferes its measurement and moreover, becomes a cause for a defect to the quality of the semiconductor device itself.
In a horizontal apparatus of the above-mentioned sort, it is necessary to make large the distance between the stage 2 and the processing unit 8 so as to permit movements of the sample stage 2 in the X and Y directions. The arm 3 which is provided on the rear side of the stage 2 or on the side opposite to the position of the processing unit 8 and carries the needle 4 and microscope 5 thereon is in the form of an elongated cantilever arm supported at a point outside the moving stroke of the stage 2. Due to this structural requirement, the arm 3 tends to undergo vibrations. Since the pattern of a semiconductor device to be measured has a minute size, for example, 200 .mu.m square on the other hand, it is essential to use a wire of an extremely fine diameter as the contact needle 4. If the arm 3 supporting the needle thereon is subjected to vibrations, the amplitude of the vibrations is magnified at the tip of the needle 4. This makes it difficult to align the tip of the needle 4 with a desired portion of the pattern.
As the distance becomes longer between the stage 2 and the processing unit 8, the lengths of lead wires 6,7 connecting them together become longer. Here, it should be noted that a probing machine of the above sort is not necessarily limited to the use of a single processing unit 8 but in many instances, uses a plurality of processing units in parallel. The lengths of wires which connect them together become substantial as a consequence. If these wires are unduly long or their lengths are different, phase shifts occur among high frequency measurement signals, leading to such problems that calibration such as zero adjustment may be rendered difficult and/or measurements may be rendered difficult due to production of noises.
When N.sub.2 gas of a low temperature is blown from the periphery of the stage 2 in a conventional apparatus of the horizontal type in order to conduct measurements at a predetermined low temperature, the low-temperature gas meets together at the center of the stage 2 and then flows upwards. The low-temperature gas may hence frost the objective lens of the microscope 5.
In the case of apparatus of the horizontal type, operator are required to take bent positions upon measurements as shown in FIG. 6. When they are required to work for many hours, their working positions may cause lumbagos. Such working positions are certainly undesirable from the viewpoint of human engineering.
In addition, there have also been known wafer probing machines in each of which a semiconductor device is measured while it is heated. As their heating means, there have been known an electric heater built in a stage on which a semiconductor device is held, a heating system in which an inert gas heated by heating means provided at a location remote from a stage is guided through a pipe line to a nozzle provided near the stage and is then blown out from the nozzle against a semiconductor device held on the stage. These conventional heating systems are also accompanied by one or more problems.
The heating means making use of an electric heater built in a stage is widely used, as a common technique, in spin processing apparatus for the coating or etching of semiconductor devices. This heating means is however accompanied by a problem that if it is employed in a probing machine, measurement results are unavoidably affected by electrical or magnetic noises generated from the electric heater and the characteristics of the semiconductor device cannot be measured accurately.
Turning next to the means for blowing a heated inert gas, this is an indirect heating system in which the heat of the electric heater is transmitted to the semiconductor device by means of the gas. This heating means is hence accompanied, for example, by the following shortcomings. The responsibility of the heating means is low and its temperature control is inaccurate. Its heating efficiency is low and a lot of energy is wasted. When an electric heater is used to heat the inert gas, the heating means is provided at a suitable position remote from the stage in order to avoid effects of electrical or magnetic noises for the same reasons as mentioned above. The pipe line for the high-temperature gas is hence long and a lot of energy is lost while the high-temperature gas is transmitted through the pipe line. A great deal of gas is required to raise the temperature of the device to a desired level because the thermal capacity of the gas is small. This is certainly uneconomical. The gas is blown out from one side of the stage and is exhausted at the opposite side. Accordingly, the device is heated to a lower temperature at the exhaust side compared with the blowing side. Therefore, the heating cannot be effected evenly.